(1) Field of the Invention
Subject invention is related the field of acoustic parametric arrays and more particularly to a new method of synthesizing arbitrary broadband signals for a parametric array.
(2) Description of the Prior Art
In its simplest form, an acoustic parametric array includes a transducer transmitting a signal composed of at least two discrete frequencies, hereinafter called "primaries". As the primaries propagate in a medium such as water, as result of medium nonlinearities, sum-and difference frequencies, hereinafter called "secondaries" are generated along the length of the primaries. At some distance, the higher frequency sounds are absorbed, leaving the difference frequency secondary as the resultant. Because the secondary is generated along the length of the primary beams, the parametric array behaves as a virtual endfire array whose length depends upon the absorption of the primaries in the medium, and which is characterized by narrow, nearly side-lobe free beams at relatively low frequency and over broad frequency bands.
Westervelt in his article; "Parametric Acoustic Array", J. Acoust. Soc. Am. 35, 535-537 (1963); first described an acoustic parametric array. A large number of researchers have studied its properties and operation which are now fairly well understood. Most of this work on parametric arrays has been directed towards narrow-band sources in which two discrete primaries interact to form a secondary with a single difference frequency as the secondary. Berktay in his article; "Possible Exploitation of Nonlinear Acoustics in Underwater Transmitting Applications", Journal of Sound Vib. 2, 435-461 (1965); first discussed the possibility of obtaining a broadband secondary signal by the use of pulsed primary signals. His assumptions limited his results to the case in which small signal absorption limited the array length to be in the nearfield of the transducer. Subsequently, a number of researchers have done both theoretical and experimental work and considered cases in which nonlinear absorption and farfield generation takes place. However, most of the previous work has been concentrated on trying to determine the secondary pressure for a given primary. The problem of finding the transmit signal which, when applied to the projector (transmitter) would result in a received waveform by the hydrophone (receiver) that is closed to a given shape or waveform has not been so far addressed. Because of the importance of low frequency directed beam in the areas of communication and oil exploration using a profile of the bottom of the ocean, it is desirable to have a technique for obtaining a low frequency directed beam pattern having a particular signal characteristics which is obtained by applying the necessary primaries.